Remote control of fluorescent lamp ballast using power flow interruption coding with means to maintain filament voltage substantially constant as the lamp voltage decreases

ABSTRACT

A dimming ballast system wherein the light output of the lamp is controlled by a remote source. The ballast output voltage is regulated by a voltage control feedback loop, the dimming function being achieved by varying the set point of the control loop. The remote control signal, coded by interrupting the current flow to the ballast for a short period, is processed digitally by a microcomputer which generates information to vary the control set point. Circuit means are provided to control the filament voltage of the fluorescent lamps such that as the lamps are dimmed down, the filament voltage is maintained or slightly increased, thus prolonging lamp life and stabilizing the lamp light output.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to remotely controlled dimmable electronicballasts for powering gas discharge lamps and, in particular, to aballast system which utilizes a power line interruption coding systemand means for controlling the lamp filament voltage such that as thelamps are dimmed, the filament voltage is increased.

2. Description of the Prior Art

The fluorescent lamp is designed to be a replacement for theincandescent lamp. The fluorescent lamp offers very large energy savingsas compared to incandescent lamps. For example, a 28 watt fluorescentlamp offers the same light as a 100 watt incandescent lamp. Thistremendous energy savings has been ignored in some applications becauseof the nonavailability of appropriate ballasts and control systems. Forexample, special lighting in restaurants, hallway lights, and otherareas wherein the light level needs to be controlled for either energysavings or special effects has in the past used incandescent lamps withenergy-wasteful dimming systems to obtain the desired effect.Incandescent dimming systems utilize either variable transformers,triacs or electronic means. The electronic means are the mostcost-effective but have serious drawbacks in the form of a very lowpower factor, low efficiency and increased harmonic generation. Theenergy that may be saved by reducing the kilowatts delivered to the lampload is utilized because of a low power factor and high harmonicgeneration. Since low power factor and high harmonics are harmful to thepower system, and power companies continue to search for ways to givetheir customers the lighting aesthetics they desire while still savingenergy.

A dimming system would require adding extra wires in the wall to connectthe controls and switches to the dimming ballasts. This is generallyunacceptable, since it is very expensive and thus the prior art soughtto communicate to the ballasts in a different manner. Prior art systemsinvolved using carrier current type communications over the power line.A well-designed carrier current type system generally will work reliablyin even difficult conditions. However, the greatest drawback with thistype of communication system is that it is a broadcast system, i.e.signals are transmitted in all directions along the wires and thereforeare required to carry complex coding information. In addition, theballasts themselves are required appropriate decoding or addressingcircuitry. The overall modification is costly and requires a much largerwall switch box to accommodate the additional equipment. In addition tothe communication problem noted, the prior art has sought to providecost efficient techniques for controlling dimming control of thefluorescent lamps. One of the approaches uses a frequency change method(shift) to both control the lamp current (power) and to maintain thelamp filament voltage substantially constant as the lamps were dimmed inorder to maintain lamp life. Frequency dimming circuitry, however, addsto the overall cost and complexity of the dimming ballast system. A lesscostly prior art technique utilizes a variable voltage power source tocontrol lamp dimming. However, filament voltage could not be controlledby a simple circuit to preserve lamp life, thus making the techniquecommercially unfeasible.

Typical of the prior art ballast control systems are those disclosed inU.S. Pat. No. 4,717,863 to Zeiler wherein a frequency modulated circuitis utilized to provide a variable voltage to dim the fluorescent lamp,an optical feedback system being utilized to regulate the frequency ofthe output signal; U.S. Pat. No. 4,523,128 to Stamm et al. whichdiscloses a system for the remote control of a solid state ballast whichinterfaces with a power line carrier system to provide externaladdressing control signals, the control system including a signalreceiver for receiving and recognizing remotely transmitted controlsignals addressed to the ballast; U.S. Pat. No. 4,889,999 to Rowen whichdiscloses a control system wherein information is transmitted toindividual dimmer controls by extra wires, the dimmer controls using atriac to control the voltage to the ballast to dim the light output;U.S. Pat. No. 4,388,567 to Yamazaki et al. which discloses a system forremotely controlling a dimming apparatus which uses single phase controlto vary the voltage and therefore control light output; U.S. Pat. No.4,388,563 to Hyltin which discloses a solid state fluorescent lampballast circuit in which line voltage is chopped to provide a highfrequency input to a fluorescent lamp, the duty cycle of the choppingswitches being modulated to permit dimming of a remotely located lamp;and U.S. Pat. No. 4,866,350 to Counts which discloses a system whereinpower is provided to a fluorescent lamp through a single integratedcircuit chip, control logic within the chip operating power switchestherein at a frequency which is optimum for the fluorescent lamp.

Although the aforementioned prior art systems provide various featureswhich improve upon the ballast used in fluorescent lighting systems,they all suffer in one way or the other from the disadvantages notedhereinabove, i.e. requiring a carrier current type encoding systemand/or lamp dimming techniques which are costly, complex and notcommercially viable.

SUMMARY OF THE PRESENT INVENTION

The present invention provides a dimming ballast system within abuilding structure wherein the lamp dimming is controlled from a remotesource, the coded control signal being generated by interrupting thenormal building power line in a predetermined sequence. In addition, thelamp filament voltage is increased or maintained as the d.c. bus voltageof the ballast is decreased as the lamp is dimmed. The ballast systemcomprises an electronic power factor correction portion which includes apower factor correction integrated circuit (IC), an inductor choke, aMOSFET transistor and a conversion portion. The MOSFET is switched onand off by a signal generated by the integrated circuit, which causesenergy transfer from the inductor choke to the ballast DC bus to providea DC voltage which is higher than the peak of the input line voltage tothe conversion portion. The switching cycles of the MOSFET arecontrolled by the integrated circuit. The integrated circuit senses theinput voltage waveform and forces the input current to closely followthe input voltage. As a result, the input current and the input voltagewill almost be in phase and the power factor will be close to 1. Thus,the harmonic components of the input current will be extremely small.The output DC voltage is regulated by means of a voltage controlfeedback loop which is determined by resistance value connected to theintegrated circuit. By varying the resistance value, the output voltagewill be changed accordingly, which in turn provides the dimmingfunction. In a preferred embodiment, a microprocessor processes theremote control signal, the digital information of the control signalbeing decoded and the proper resistance value then being selected.

The output section portion of the ballast system comprises aself-resonating half-bridge invertor. To control the filament voltage, aferrite core is used. The filament voltage is determined by the numberof turns wound on the core and the frequency of the resonating circuit.As the output voltage is reduced, the frequency of the resonatingcircuit will increase as lamp impedance increases, thus increasing thefilament voltage of the lamp accordingly. This later feature stabilizesthe lamp light output when dimmed and prolongs the life of the lamp.

By interrupting the power line to provide the signal coding necessaryfor remote control of lamp dimming, the use of high frequency modulatedsignals, typically used in the prior art, is eliminated. This in turneliminates the "broadcast" characteristics found in the prior artsystems, thus reducing system cost and complexity since additional wiresand addressing circuitry are not required. The concept of encoding thepower signal emanating from one part of a building structure to controla dimming ballast located in another part of the building without addingadditional wires as described hereinabove can be utilized to controldevices other than dimming ballasts, such as gas discharge lamp sources,air conditioners, and dampers.

DESCRIPTION OF THE DRAWING

For a better understanding of the invention as well as other objects andfurther features thereof, reference is made to the following descriptionwhich is to be read in conjunction with the accompanying drawingwherein:

FIG. 1 is a block diagram of the system of the present invention;

FIGS. 2(a) and 2(b) are waveforms to illustrate the missing pulse codingsystem of the present invention;

FIG. 3 is a schematic diagram of the transmitter portion of the ballastsystem of the present invention;

FIG. 4 is a flow chart illustrating the operation of the encodingmicroprocessor;

FIG. 5 is a schematic diagram of the dimming ballast portion of theballast system of the present invention; and

FIG. 6 is a flow chart for the microprocessor utilized to decode theinformation transmitted from the wall switch box.

DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, a simplified block diagram of the dimmingballast control system of the present invention is illustrated.

In particular, standard electrical power (standard line source,typically 120 volts AC, 60 hertz) is supplied over power lines 10 and 12located in a building structure to a controller 14 (waveform A, FIG. 2A)located in a wall switch unit. Controller 14, as set forth in moredetail hereinafter, removes (or reduces the amplitude of) one pulse fromthe incoming alternating waveform in a predetermined time period N(waveform B, FIG. 2A). The time between missing pulses corresponds to adesired lamp dimming (power) level. The output signal from controller 14is applied to a plurality of remote ballast units 16₁, 16₂ . . . 16_(n).Each ballast unit is identical and thus only unit 16₁ will be describedin detail. The ballast unit 16₁ comprises a rectifier 18 which providesa rectified DC voltage at its output (waveform C shown in FIG. 2A), adecoder unit 20 coupled to the output of rectifier 18, decoder 20providing a reference control signal on lead 22 to IC regulator andpower factor control unit 24 as will be described in more detailhereinafter. Regulator unit 24, coupled to the output of rectifier 18,provides a regulated DC voltage output which is adjustable. The outputsignal from regulator unit 24 is coupled to invertor 26 which provides ahigh frequency AC signal, the frequency of the signal being dependent onthe magnitude of the DC signal at the invertor input. The high frequencyAC signal is coupled to ballast capacitor 28 and then to the fluorescentlamp being controlled. The dimming ballast systems are located remotelyfrom the wall switch and typically adjacent the fluorescent lamps.

An alternate encoding arrangement is represented by the correspondingwaveforms shown in FIG. 2B wherein a sequence of missing pulsescorresponds to a particular dimming level desired. The followingdescription assumes that the encoding system shown in FIG. 2A is beingutilized.

As will be explained in detail hereinafter, the output signal from thedecoders determine the magnitude of the DC applied to invertor 26 andthus the AC voltage (current) applied to lamp 30. Invertor 26, inaddition, controls the lamp filaments in a manner such that the filamentvoltage increases proportionately to the decrease (dimming) in voltageapplied across the lamps.

In the system illustrated, the same dimming voltage is applied to eachlamp responding to controller 14. Typically, a single wall switch in aroom, for example, controls all the lamps in that room in an identicalfashion.

FIG. 3 is a schematic diagram of controller 14. The voltage on lines 10and 12 is applied to the primary winding 30 of transformer T1. Anauxiliary winding 32 of transformer T1 is connected to a full waverectifier 34, the output thereof being coupled to pin VCC ofmicrocomputer 36 via power signal conditioner circuit 35. Auxiliarywinding 37 power applies the AC line signal appearing at primary winding30 to pin Tl of microprocessor 36 in the form of periodic input pulses.Preferably, microcomputer 36 is the Model 8048 manufactured by IntelCorporation, Santa Clara, Calif. Input pins P₁₀, P₁₁ . . . P₁₅, areconnected to ground via keys, or pushbuttons, 50, 48 . . . 40,respectively, as illustrated. Keys 40, 42 . . . 48, shown in the open,or inoperative, position turn on the fluorescent lamps when pressed toclose the contacts and correspond to five different lamp power settings.The keys are located in the wall switch box. Key 50 corresponds to theoff key and when closed causes microcomputer 36 to open J2 and turn offthe lamps. As an optional device, a remote infrared controller 54 isused to control an infrared receiver 56, the coded output thereof beingconnected to pin To of microprocessor 36. As will be described withreference to the flowchart shown in FIG. 4, activation of one of thepushbuttons 40, 42 . . . 48 generates a signal at pin P₂₂ which controlsthe magnitude of the voltage at the output of transistor Ql, and throughtransformer T2, the conducting state of thyristor, or silicon controlledrectifier, 60. Thyristor 60 determines the time period N between missingpulses of the coded signal (waveform C, FIG. 2A) being transmitted todecoder unit 20.

FIG. 4 illustrates the transmitter encoding flow chart. The flow chartis set forth to enable a computer programmer to program the Intelmicrocomputer described hereinabove in a manner such that theappropriate dimming coded signal is produced in response to a selectedkey. In particular, when the microcomputer 36 is activated (symbol 70),the microcomputer initially determines whether any one of the keys 40,42...48 have been engaged (symbol 72). If one of the coding keys hasbeen depressed, the microcomputer next determines whether the turn-offkey has been activated (symbol 74). If yes, the system is turned off(symbol 76) and the microcomputer returned to the start position. If noturn-off signal is present, the microcomputer 36 checks to see if relayJ2 is activated (transistor Q3 is turned on) (symbol 77). If not, Q3 isturned on and a delay (symbol 81) is imposed to enable the lamps tostart at the highest level (intensity) before returning to its presetlevel. If the microcomputer determines that relay J2 is engaged (Q3 on),the microprocessor 36 next searches a particular address in a look-uptable for the depressed key (symbol 78). It should be noted that relayJl is used to minimize energy losses during the time periods when theremote control function is not being utilized. When the system is used,the J1 relay contact is in the open position. After the relay contact isin the open position (symbol 80), thyristor 60 is used to prevent thefirst pulse (waveform FIG. 2A) in the AC signal to be transmitted to theballast (symbol 82). As noted above, a register in microprocessor 36 hasbeen set to a value corresponding to the particular key which has beendepressed (in fact, the value in the register corresponds to the timeperiod N). The register (symbol 84) is decremented each time a rectifiedpulse is detected at pin T₁ of microcomputer 36. If the register is notzero (symbol 86), the output at pin P₂₂ is such that transistor Q₁causes thyristor 60 to allow the power to be transmitted to the ballastvia lines 13 and 15. When the register is zero, the output at pin P₂₂causes transistor Q₁ to bias thyristor 60 in a manner to prevent thesecond pulse to be transmitted during the period N (symbol 88). Therelay is then deenergized (symbol 90), closing the relay contact. Aftera predetermined delay (symbol 92) to allow for mechanical "debouncing"of the keys, the cycle repeats itself (microprocessor 36 scans pins P₁₀,P₁₁ . . . P₁₅ continuously to ascertain whether the setting for the lampintensity has changed).

As noted previously, the ballast system of the present invention isarranged to have a remote infrared control option whereby a user canadjust the lamp dimming without depressing a key on the wall switch box.In this case, if the microprocessor 36 determines a key has not beendepressed (symbol 72), a determination is made if a signal is at pin To(symbol 94). If not, the cycle is restarted. If there is a signalpresent, a check is made to see if the signal (most remote infraredsignaling devices have a preset address data) has a correct address(symbol 96). If not the cycle is restarted. If the address is correct, acheck is made to see if the preset signal data (a portion of the entiredata) is correct (symbol 98). If not, the cycle restarts. If the data iscorrect, the remaining portion of the data, coded to correspond to oneof the keys 40, 42 . . . 48 when depressed, causes the cycle (symbol100) to start at the input point to symbol 74 as illustrated.

Referring now to FIG. 5, a schematic diagram of the ballast system P₂₃is illustrated.

The coded signal (waveform B, FIG. 2A) on the power output lines 13 and15 (FIG. 3) is coupled to the input power lines 63 and 65, respectively,as illustrated. The coded signal is applied to rectifier circuit 18comprising diode full wave bridge circuit 67 and to regulator unit 24.The signal output from bridge circuit 67 is coupled to Schmitt triggercircuit 71 via a voltage divider circuit comprising resistors 73 and 75,trigger circuit 71 converting the missing rectified AC pulse waveform toa corresponding shaped pulse waveform which is applied to input pin 30of microprocessor 120. The output of the bridge circuit 67 is alsoapplied to one input of integrated circuit 110. As explainedhereinafter, the output of circuit 110, preferably a commerciallyavailable Siemens TDA 4814A chip, switches MOSFET 112 from a conductingto a non-conducting state and vice versa at a frequency rate dependentupon the magnitude of the input voltage, the DC bus voltage, theinductance value of the inductor choke (114) and the desired inputcurrent. This in turn causes energy to be transferred from inductorchoke 114 to output junction 118 (and across capacitor C2). Integratedcircuit 110 also senses the input voltage waveform at pin 11 and forcesthe input current to resemble the voltage. As a result, the inputcurrent and input voltage will be substantially in phase and the powerfactor (cosine of the phase angle between the waveforms) will be closeto one, typically 0.995. Thus, the harmonic content of the input currentwill be greatly minimized. As noted above, microcomputer 120, preferablyan Intel 8051, functions to decode the input coded signal andeffectively generate a resistance value at node 116 (input to pin 12 ofchip 110) corresponding to the appropriate key depressed in the wallswitch unit (it should be noted that the ballast system of the presentinvention can also be utilized without remote control i.e. if a variableresistance is applied directly across the taps a and b illustrated). Inparticular, and as explained with reference to the flow chart shown inFIG. 6, microcomputer 120 continuously scans input pin P₃₀ anddetermines the length of time between missing pulses. According to thisinformation, selected ones of the open drain inverters 124, 126...130connected to pins P₁₀, P₁₁ . . . P₁₃, respectively, are biased into thenon-conducting state, thus connecting the resistances associatedtherewith into a voltage dividing circuit with resistances R120. Thevalue of the resistance applied to pin 12 determines the DC bus voltageat junction 118. Changing the DC bus voltage at junction 118 determinesthe energy transfer (pulse) rate of the signal from inductor (choke) 114applied across capacitor C2. It should be noted that chip 110 and theboost converter circuit are connected in a manner such that a total DCvoltage at junction point 118 is greater than the peak input voltage(170 volts) applied to lines 63 and 65.

The output section of the ballast is basically a self-resonatinghalfbridge invertor which converts the DC power to high frequency AC(20-50 KHZ), the circuit comprising capacitors C3, C4, C6, C7 and C10,transistors Q1 and Q2, transformer T103 and diodes D5, D6, D7, andresistors R1, R2, R3 and R4. In this invertor circuit, the outputvoltage waveform is close to sinusoidal. The frequency is mainlydetermined by capacitor C7, the inductance of the primary winding oftransformer T103, capacitor C10 and load (lamp) impedance. To controlthe filament voltage applied to fluorescent lamps L1 and L2 (althoughonly two lamps are illustrated, the concept of the present invention canbe utilized with one or more than two fluorescent lamps), a transformerT104 which is a ferrite core is utilized. Increasing filament voltageduring dimming is accomplished by saturable transformer T104 which isconnected in series with the secondary winding of T103 and capacitor C7,a resonant tank with reasonably higher impedance than the inputimpedance of T104. The filament voltage is determined by the number ofturns wound on the ferrite core and the frequency of the resonatingcircuit. Transformer 104 is designed to operate at deep saturation whenmaximum voltage appears across C2. As the output voltage across C2 isreduced, for example, by the control of microcomputer 120, the frequencyof the resonating circuit will increase as the impedance of thefluorescent lamps increase. Thus, the filament voltage of the lamps willincrease accordingly, both stabilizing the lamp light output when dimmeddown and increasing lamp life.

The operating points of the fluorescent lamps in the preferredembodiment are set substantially as follows:

    ______________________________________                                                              Resonant                                                Light Output                                                                            Lamp Voltage                                                                              Frequency Filament Voltage                              (%)   Watts   (Volts)     (KHz)   (Volts)                                     ______________________________________                                        100%  230     420         20.04   3.54                                         75%  172     336         20.62   3.72                                         50%  115     267         22.10   3.94                                         25%   57.5   237.5       24.57   4.095                                        10%   23     220         25.55   4.072                                       ______________________________________                                    

The details of the circuit operation are as follows. The voltage outputfrom the secondary winding of T104 is applied across lamps L1 and L2.The inductor (choke) T102 limits current flow in the circuit, thusenabling a sinusoidal waveform to be generated. The voltage acrosswindings T103 and T104 also have a sinusoidal waveshape, winding T104being coupled to the lamp filament windings, the secondary winding ofT103 being coupled across the lamp to initiate the arc. Windings T103and T104 and capacitor C7 form the circuit resonant tank. Winding T104is designed to make the core saturate, the secondary voltage from T104being substantially constant because its flux density is set to themaximum. The nominal (highest level) DC bus voltage at junction point118, for 100% light output, is set at 420 volts. When the DC voltage atjunction point 118 is reduced, the voltage of the secondary winding ofT103 is also reduced proportionately. Thus the current through C10 andthe lamps Ll and L2 is also reduced. Since each lamp has a negativeresistance, as current is reduced, the voltage increases and the lampimpedance increases. The effective capacitance of C10 reflected to theprimary of T103 is reduced, the overall circuit reactance thus beinglower and increasing the circuit resonant frequency. Since the core ofT104 is in deep saturation, the voltage reduction does not change itsstatus, the flux density remaining substantially constant. In this case,the voltage applied to the lamp filaments is essentially proportional tothe resonant frequency. The above operation repeats itself as thedimming voltage decreases.

Resistances R100 and R101 sample the sinusoidal input voltage waveformto the circuit, or chip, 110 to control the power correction factor. Itis, as noted hereinabove, preferred to have the input current close tothe input voltage (phase, shape). The input current is sensed as itflows through resistors R107 and R108. Resistors R105 and R106 are usedto compensate for the voltage of the fluorescent lamps being used.Capacitor C108, resistor R120 and the effective variable resistance atnode point 116 functions both to filter out 120 Hz ripple and as avoltage control loop, i.e. to stabilize and regulate the DC voltage atnode point 118. FIG. 6 illustrates the receiver decoding flow chart. Theflow chart is set forth to enable a computer programmer to program theIntel No. 8051 microcomputer described hereinabove in a manner such thatthe D.C. voltage corresponding to the desired lamp output dimming isprovided to the circuitry controlling lamp operation.

At the start of the operation of microcomputer 120 (FIG. 5), register R₀is set to zero (symbol 152) and the input to microcomputer 120 from theSchmitt triggers is tested (checked) continuously (symbol 154). Theinput testing is done on a continual basis. If the initial input testindicates that an input pulse is not present, the R₀ register isincremented one unit (symbol 156). If the input test indicates a pulseis present, the process restarts. The input is tested again (symbol 158)and if no input pulse is present, the R₀ register is incremented oneunit. If the input test indicates that a pulse is present, the count inthe R₀ register is compared with the value in the PULSE register (symbol160). The PULSE register has a value corresponding to the time period ofa missing pulse. If the value in the R₀ register corresponds to thevalue in the PULSE register (corresponding to the first pulse after aperiod during which no pulse appeared) a third register (CODE) is testedto ascertain if it is set to 1 (symbol 162). If not, the CODE registeris set to 1, the timer is first cleared and then started (symbol 164).If the CODE register previously has been set to 1, the value in thetimer is transferred to an accumulator, the value in the accumulatorcorresponding to an address in a look-up table (symbol 166). The valuein the lookup table corresponds to the key depressed in the wall switchbox. If the value in the timer is not the preset value (symbol 168), thetimer value will not be decoded, the CODE register is cleared and thetimer is stopped (symbol 172) and the process restarted. If the value inthe timer is correct, a control code corresponding to the key depressedin the wall switch box can be obtained from the look-up table. Thecontrol code is sent to pins P₁₀ through P₁₅ to control thecorresponding pin (symbol 170). In this case, the CODE register iscleared, the timer stopped and the process restarted.

The present invention thus provides an improved dimmable fluorescentlamp ballast system which encodes the power line signal in a buildingstructure in accordance with a desired dimming state such thatadditional wires are not required in the structure, thereby reducing thecost of system installation. In addition, the lamp filament voltage ismaintained substantially constant or increased slightly as the dimmingvoltage is decreased, thus both stabilizing lamp light output andprolonging lamp life.

The concept of providing a building structure control system forcontrolling the operating status of a remote device, as set forth in thepresent invention, without adding additional wires to the structure, canbe utilized to control devices other than fluorescent lamp ballasts,such as other gas discharge lamp systems, air conditioners, and dampers.

While the invention has been described with reference to its preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the true spirit and scope of theinvention. In addition, many modifications may be made to adapt aparticular situation or material to the teaching of the inventionwithout departing from its essential teachings.

What is claimed is:
 1. A dimmable fluorescent lamp ballast systemcomprising:means for providing a first AC voltage signal; meansresponsive to said first AC voltage signal for providing a DC voltagesignal having a magnitude which is adjustable between first and secondlevels, said first level being higher than said second level; afluorescent lamp having filament means associated therewith; firstcircuit means responsive to said first level DC voltage signal forgenerating a second AC voltage signal having a magnitude proportional tothe magnitude of said first level DC voltage signal, said second ACvoltage signal being coupled to said fluorescent lamp; and secondcircuit means responsive to said first level DC voltage signal forgenerating a third AC voltage signal having a magnitude proportional tothe resonant frequency of said second circuit means, said third ACvoltage signal being applied to said lamp filament means, the magnitudeof said second AC voltage signal decreasing as said DC voltage signal isadjusted to said second level from said first level, the magnitude ofsaid third AC voltage signal when said DC voltage signal attains saidsecond level remaining at a value at least equal to its value when saidDC voltage signal is at said first level.
 2. The system as defined inclaim 1 wherein said first and second levels correspond to first andsecond light outputs, respectively, from said fluorescent lamp.
 3. Thesystem as defined in claim 1 wherein said DC voltage signal isadjustable to levels between said first and second levels.
 4. The systemof claim 3 wherein said adjustable light levels correspond to selectedlight outputs from said fluorescent lamp.
 5. The system of claim 1wherein said first AC voltage signal is coded to correspond to eithersaid first or second selected light output from said fluorescent lamp.6. The system of claim 5 wherein said voltage signal responsive meanscomprises decoder means for providing a resistance value correspondingto the coded value of said first AC voltage signal.
 7. The system ofclaim 6 wherein said decoder means comprises a microcomputer.